1. Technical Field of the Invention
The present invention relates generally to a hydraulic control system for an automatic transmission of an automotive vehicle, and more particularly to an improved hydraulic control system which is so designed as to absorb mechanical shifting shocks effectively, caused by engagement operations of clutches and brakes of an automatic transmission, without use of shock-absorbing accumulators.
2. Background Art
Reference will be made to FIG. 18 to describe a structure of a conventional automatic transmission system.
The shown automatic transmission system includes generally a power train 200 and a hydraulic control system 300. The hydraulic control system 300 includes an oil strainer 151, an oil pump 152, a primary regulator valve 153, a secondary regulator valve 154, a pressure relief valve 155, a throttle modulator valve 156, a first accumulator control valve 157, a second accumulator control valve 158, a down-shift plug 159, a throttle valve 160, a cutback valve 161, a lock-up solenoid 162, a lock-up relay valve 163, a cooler by-pass valve 164, an oil cooler 165, an 1-2 shift valve 166, a 2-3 shift valve 167, a 3-4 shift valve 168, a low coast modulator valve 169, a second coast modulator valve 170, a B1 orifice control valve 171, a second lock valve 172, a first solenoid 173, a second solenoid 174, and a manual valve 175.
The automatic transmission system further includes five hydraulic dampers or accumulators: a C1 accumulator 176, a C2 accumulator 177, a B2 accumulator 178, a C0 accumulator 179, and a B0 accumulator 180 for absorbing uncomfortable mechanical shocks caused by engagement of clutches and brakes of the power train 200.
The power train 200, as shown in FIG. 3, includes three clutches: an overdrive clutch C0, a forward clutch C1, and a direct clutch C2 and four brakes: an overdrive brake B0, a second coast brake B1, a second brake B2, and a first and reverse brake B3.
The overdrive clutch C0 is responsive to hydraulic pressure supplied to its actuator (not shown) to engage a carrier 182 of an overdrive planetary gear set 181 with its sun gear 183. The forward clutch C1 serves to engage an input shaft 184 with a ring gear 186 of a front planetary gear set 185. The direct clutch C2 engages the input shaft 184 with a sun gear 187 of the front planetary gear set 185. The sun gear 187 extends into a rear planetary gear set 188 to work as a sun gear thereof.
The overdrive brake B0 is responsive to hydraulic pressure applied to its actuator (not shown) to engage the sun gear 183 of the overdrive planetary gear set 181 with a stationary transmission casing 189 for locking rotation of the sun gear 183. The second coast brake B1 locks the common sun gear 187. The second brake B2 locks rotation of an outer race (not shown) of a one-way clutch F1. The first and reverse brake B3 locks rotation of a carrier 190 of the rear planetary gear set 188. An intermediate shaft 191 is arranged to connect a carrier 192 of the front planetary gear set 185 with the carrier 182 of the overdrive planetary gear set 181.
The power train 200 uses three one-way clutches F0, F1, and F2. The clutch F0 is called an overdrive one-way clutch which restricts a counterclockwise rotation of the carrier 182 relative to the sun gear 183 of the overdrive planetary gear set 181. The clutch F1 restricts a counterclockwise rotation of the common sun gear 187 upon activation of the second brake B2. The clutch F2 serves to lock a counterclockwise rotation of the carrier 190 of the rear planetary gear 188. The power train 200 further includes a counter drive gear 193, a torque converter TC, and a lock-up clutch LC.
The above mentioned conventional automatic transmission system includes, as can be seen in FIG. 18, in the hydraulic control system mechanical directional control valves such as the solenoid valves 173 and 174 which serve as pilot valves. In addition, the hydraulic pressure supplied from the oil pump is regulated only by mechanical valves such as the pressure regulator valves 153 and 154. The engagement of the lock-up clutch LC for direct connection between a pump and a turbine of the torque converter is achieved by use of a mechanical spool valve such as the lock-up relay valve 163 controlled by the solenoid valve 162 working as a pilot valve. Further, the accumulators 176 to 180 and the accumulator control valves 157 and 158 are arranged in the hydraulic circuit to reduce mechanical shocks upon engagement of the clutches and the brakes. These arrangements, as also apparent from the drawing, make the hydraulic control system complex and bulky, causing the whole transmission system to become heavy as well as leading to the increase in manufacturing cost.